1. Technical Field of the Invention
The present invention relates generally to the technical field of particle accelerator design and engineering and, in particular, to a method for controlling and correcting energy correlations in charged particle beams for high performance linear accelerators.
2. Description of the Related Art
Short, subpicosecond pulses are central to many of the next generation light source initiatives that are typical of linear accelerators. The free electron laser (FEL) is considered to be the main candidate for future short wavelength (UV to X-ray), short pulse (femto- to attosecond) light sources. Demands on the electron beam needed to drive this class of FELs are very challenging with present technologies. In particular, a small intrinsic chirp is present at the output of the last bunch compression stage of the linac to compensate for wakefield effects through the rest of the accelerator. It is required that this energy spread be compensated using a specially designed device.
A key consideration for linac optimization, as an example, for the UK NLS (Soft X-Ray FEL), Berkeley XUV plasma accelerator driven concept etc., is to reduce the final energy chirp on the electron bunch in the FEL train, ideally keeping it below the intrinsic SASE bandwidth. This requirement is particularly challenging for L-Band linacs, where the wakefields are reduced compared to normal-conducting S-Band linacs and cannot be used to remove the energy chirp. Attempts to operate the RF cavities beyond-crest after the final bunch compressor have also proved ineffective due to the short bunch length. As such, the optimization has been carried out in such a way as to minimize the initial size of energy chirp imprinted on the beam by running the RF cavities close to on-crest, and compensating for the reduced energy chirp with an increase in bunch compressor strength. However, in this scheme care has to be taken in order to avoid increasing the sensitivity of the linac to jitter.
One common method would be the use of the beam's self-wakefield in the linac itself to correct the energy spread. The reduced wakes of the SRF linac do not allow compensation of the energy spread that is left over at the output of the last compressor for wakefield effects through the rest of the accelerating stage. Another approach is the use of an extra powered rf cavity phased so that the beam experiences a zero-crossing of the rf so that the chirp is removed. This active technique requires an additional cavity and rf power source, possibly at a different frequency from the linac rf. While workable, this solution adds considerably to the overall cost of the linac.
In such embodiments, the use of existing technologies to eliminate the energy chirp involves increased cost and complexity of the system. This is particularly problematic for the use of XFELs as turnkey research instruments. Given the anticipated user demand for these facilities, system reliability is an important consideration. The use of a passive energy correction technique as presented here improves the system performance without impacting reliability unlike existing technologies.
Further, present technology methods for beam energy correction lack flexibility in adapting to a range of beam lengths and energy profiles, required for optimization of system performance.
Further still, the paucity of control options in existing compensation schemes impacts maintenance of long term system stability and hence accuracy and replicability of user measurements.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Consequently, a need has been felt for providing an apparatus and method for compensating and controlling the energy spread in a charged particle beam by spatially redistributing the energy within the beam.